Science —

CLEO Munich: Superoscillation edition

Wherein I wax lyrical about learning of a phenomenon completely new to me: …

Sometimes it is just irresistible: all the geeks who play with lasers get together in Munich and talk about their favorite colors, compare burns, and drink beer out of one liter steins. Now, the beer is the most important part of the week, but describing a 65kg American graduate student having their first experience with German beer gets old after a while. So, instead, I thought I would regale you with tales of... lasers, light, and other fascinating bits of science.

So, in the first of what will be a series, I will provide a few reports on some of the things that caught my eye over the course of the week long CLEO meeting (Conference on Lasers and Electro-Optics, combined with the European Quantum Electronics Conference). First up is something called superoscillation. If you don't know what that is, you are not alone—it is the one thing at the conference that I had never heard of before.

Here is the basic idea: take a signal—it could be optical, electrical, or sonic, doesn't really matter. This signal will have a power spectrum that spans a certain frequency range. However, an analysis of the signal at particular time intervals might show that there are frequency components outside this frequency range. These are called superoscillations.

The presenter, Sir Michael Berry, gave a great example. Beethoven's Ninth Symphony requires a signal that can support a 20kHz bandwidth but, using superoscillations, it could be encoded in a signal that has a 1Hz bandwidth. The caveat is that the power in the encoding superoscillations is e109 times smaller than the 1Hz carrier wave. So, it is pretty clear that there are limits to practical uses of superoscillations.

Berry was mainly concerned with the mathematical properties of superoscillations, rather than their pratical use—he is a theoretical physicists, so we can, if we try hard, forgive him for that. Nevertheless, he is trying to figure out how one can make use of superoscillations to overcome the diffraction limit of imaging systems.

The idea is that a normal lens will focus light to a disc that is determined by the range of spatial frequencies that the lens and the light can support. There are, literally, no higher frequency components, which is why one cannot see any features smaller than this disc size. Superoscillation provides an alternative. If one can construct a light field that has superoscillations concentrated in a small disc, then they might be used to see smaller features.

The great thing about such a scheme would be that these could be focused to arbitrarily small sizes at long distances from the lens simply by adding higher frequency superoscillations. This could be important, because the superlens schemes that are used for these applications make use of a non-propagating field, called the evanescent field, which only exists very close to the lens and precludes things like imaging the interior of a living cell.

The downside is that these superoscillations carry very, very little power, which also means that, when used to image, most of the power is carried in the low frequency, large scale, components. Clearly, there is going to have to be some very clever technological developments to enable these low frequency components to be removed so that the underlying detail can be revealed.

Berry also gave a very cool outline of an experiment that would test for the existence of superoscillations as he described them. The idea would be to produce a laser beam with a very tiny dark spot at some location. Now, the presence of superoscillations would mean that an occasional burst of photons should appear in the dark spot. An atom sitting in the dark spot would get a huge momentum kick over the course of this photon burst, so it should be detectable.

There are some problems with this idea: for instance, even trapped atoms have a fair bit of motion, so keeping them in the dark spot would be difficult. Despite this difficulty, it is an elegant idea, and a few modifications could turn it into a very cool experiment.

Superoscillations have been keeping me awake at night. I love new ideas.

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Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He is delocalised, living and working in Eindhoven and Enschede, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku